A man said to be a member of Bernard Madoff's "inner circle" has agreed to plead guilty to helping the swindler cook the books, US federal prosecutors said.

Eric Lipkin, a second-generation Madoff employee who worked at his secretive investment advisory business for 16 years, will appear in court on Monday to admit his role in the epic fraud, prosecutors said in a letter to US District Judge Laura Taylor Swain.

The letter said Lipkin would plead guilty to a six-count criminal complaint that will be submitted to the court and will include charges of conspiracy, bank fraud and falsifying various financial records.

Lipkin is entering the plea as part of a cooperation agreement with prosecutors, but the letter did not specify what sort of assistance he has been asked to provide.

In a civil lawsuit filed last year, the court-appointed trustee trying to unwind Madoff's fraud called Lipkin a member of the schemer's "inner circle" who profited handsomely from the fraud and was an active part of attempts to cover it up.

Lipkin's father, Irwin, was one of the first people hired by Madoff when he started his investment business, and was one of the key workers who began building the firm from scratch in 1964, according to the lawsuit. Eric Lipkin followed his father into the company in 1992 and acted as a "lieutenant" to Madoff, as well as the company's payroll manager.

Court trustee Irving Picard accused both father and son of being aware of the Ponzi scheme and helping to conceal it, while pocketing about 9.2 million US dollars in customer money through fictitious stock trades.

If he goes through with the guilty plea, Lipkin, who lives in Ridgewood, New Jersey, will be the third person who worked for Madoff to plead guilty to a crime.

Madoff's independent auditor and his long-time deputy have both previously admitted wrongdoing. Five other people who worked for the firm have been charged but pleaded not guilty.

Madoff, 73, is serving a 150-year prison term after pleading guilty in 2009. At the time, he claimed he acted alone.